An optical measurement method using flow cytometry (flow cytometer) is used for analysis of biological microparticles such as cells, microorganisms, or liposomes. The flow cytometer is an apparatus for irradiating the microparticles flowing through a flow channel formed in a flow cell, a microchip, or the like, with light, and detecting and analyzing fluorescence or scattered light emitted from each microparticle.
For example, for detection of fluorescence emitted from the microparticle, such as the cell or bead, using the flow cytometer, each microparticle is irradiated with light, as excitation light, having a specific wavelength at a predetermined intensity. After the fluorescence having been emitted from each microparticle is collected by a lens or the like, a filter, a dichroic mirror, or the like is employed to separate and select a target wavelengh, and the light is detected by a light receiving element such as a photomultiplier tube (PMT).
In the flow cytometer, a laminar flow is formed for passing the microparticles through substantially the center of the flow channel, but individual microparticles may have a variation in flowing position. Different flowing positions of respective microparticles bring about change in positional relationship between the microparticles and an optical system for light irradiation or light detection, detected intensity of the fluorescence or the scattered light is changed, and data accuracy is deteriorated. Therefore, a technique for detecting the flowing position of the microparticle for improving detection accuracy has been proposed (e.g., see Patent Documents 1 and 2).
In a fluid particle analyzing apparatus of Patent Document 1, detected light (scattered light) extracted from forward scattered light, sideward scattered light, or backward scattered light through a beam splitter, is detected by a quadrant photodiode, area charge coupled device (CCD), or the like. Misalignment between the center of excitation light and the center of a sheath flow is detected based on a detected position, and the position of a flow cell is adjusted so that the misalignment is within a predetermined range.
Further, Patent Document 2 discloses a technique for adjusting the focal position of excitation light or the position of a flow cell or a microchip by detecting positional information of a microparticle, using change in polarization angle generated in scattered light from the microparticle.